Phthalic anhydride is an important commercial chemical useful in the manufacture of dyes and resins. Phthalic anhydride is also an intermediate compound used principally in the manufacture of plasticizers, polyesters and alkyd resins. The color properties of this material are particularly important, especially when the phthalic anhydride is used in the production of alkyd resins, these latter materials being used as coatings in the finishes of automobiles, refrigerators, etc. The plasticizers are of two types: diesters of a monohydric alcohol, e.g., dibutyl phthalate, and mixed esters of two monohydride alcohols.
The fastest-growing use of phthalic anhydride is in the production of unsaturated polyester resins, which are the products of polycondensation reactions between mole equivalents of certain dicarboxylic acids and glycols. Resins based on phthalic anhydride are used widely in the construction, marine, and synthetic-marble industries. In most cases, the resins contain mineral or glass fibers which provide the required structural strength.
Phthalic anhydride is typically produced from raw materials such as orthoxylene (o-xylene), petroleum naphthalene, and coal-tar naphthalene. The price of these raw materials and, as a direct result, the price of phthalic anhydride have fluctuated greatly depending upon supply and demand. Because the cost of the raw materials are a major factor in the price of phthalic anhydride it is of great importance that any system used to produce phthalic anhydride capture as much of the resultant product as possible.
Phthalic anhydride can be successfully produced from any of a number of processes, i.e., (1) air oxidation of o-xylene in fixed-bed reactors, (2) air oxidation of petroleum or coal tar naphthalene in fixed-bed reactors, (3) fluid bed oxidation of o-xylene, (4) fluid bed oxidation of petroleum or coal tar naphthalene, and (5) liquid phase oxidation of o-xylene or naphthalene.
The general process scheme for the various vapor phase routes is to mix the hydrocarbon feed (in the vapor form) with compressed air and to feed the mixture to fixed-bed reactors which contain tubes packed with catalysts, e.g., vanadium oxide and titanium dioxide coated on an inert, nonporous carrier. When fluid bed reactors are used, the hydrocarbon feed in liquid form is injected directly into the fluidized bed so that the air and the hydrocarbon are mixed in the reactor. The reactors are equipped with means for removing the heat of the oxidation reactions that occur. The heat that is removed is used to generate steam.
In fluid bed reactors, provisions are made to cool the reaction mixture immediately after it leaves the reaction zone. This operation, called "quenching," is done to stop the reactions and to prevent "after-burning."
After the product stream exits either the fixed-bed or fluid bed reactors, it is cooled to cause the phthalic anhydride to condense. This allows separation of the phthalic anhydride from the gas stream. In fixed-bed systems, the phthalic anhydride is typically condensed as a solid. However, a two-stage condensation system can be used to first condense a portion of the phthalic anhydride as a liquid and then to condense the remainder as a solid.
Switch condensers that operate alternately on a cooling cycle and a heating cycle are used to collect the phthalic anhydride as a solid. The solid is then melted for removal from the condensers.
The crude phthalic anhydride is usually heat-treated in a decomposer, but in some cases chemical treatment is also used. The heat treatment is carried out by holding the molten crude at an elevated temperature [approximately 500.degree. F.] for a period of 12-36 hours and usually under a small vacuum. The purpose of the heat treatment is to dehydrate any phthalic acid in the crude to phthalic anhydride, to boil off materials such as water, and to form either condensation or volatile products with the other impurities so that the subsequent product purification by distillation is simplified.
After distillation, the pure molten product may be solidified, flaked, bagged, and stored in a warehouse. Alternatively, the molten product may be pumped into large storage tanks and then into tank cars for shipment.
One conventional system for producing phthalic anhydride from o-xylene by air oxidation is set forth in FIG. 1, attached hereto.
The process depicted in FIG. 1 is initiated by pumping o-xylene from storage tank 100 via pump 102 through filter 104 and o-xylene preheater 106, where it is heated almost to the vaporization point. Air is passed through air filter 108, silencers 110 and 114, compressed in air compressor 112, and heated in air preheater 116 before being mixed with the hot liquid o-xylene. The hot liquid o-xylene is injected into the hot air stream via spray nozzles (not shown) and vaporized. The vaporized air-xylene mixture passes through knock-out drums 118. The treated air-xylene mixture then enters fixed-bed reactors 124 and 126. Reactors 124 and 126 each contain vertical tubes packed with various catalysts. The heat of reaction is removed by molten salt circulating in the reactor shell via salt bath cooler and agitator systems 128 and 130, respectively. The salt is cooled by steam coils 132 and 134, which produce steam to be used throughout other parts of the process.
The reaction gas is composed of nitrogen, oxygen, water, carbon dioxide, carbon monoxide, argon, phthalic anhydride, maleic anhydride, maleic acid, o-toluic acid, and partial oxidation products, for example, phthalide, etc.
The reaction gases from reactors 124 and 126 enter gas cooler 136 and secondary gas cooler 138 before being conveyed to switch condensers 140. There are typically a minimum of three switch condensers 140, i.e., two of the switch condensers are on a loading (cooling) cycle and one is on a melting (heating) cycle. The crude phthalic anhydride desublimates onto tubes 141 of the switch condenser, and the remaining gas mixture exits from the switch condenser. The off-gas is conveyed to thermal oxidizers 142 and 144 for incineration.
Switch condensers 140 are cooled by passing low viscosity oil through tubes 141. The cold oil is cooled in a cold oil system by cooling water. In cooler climates, air cooling can be used.
During the heating cycle, hot oil is circulated through tubes 141 of switch condenser 140 to melt the crude phthalic anhydride plated thereon. The liquid anhydride flows into crude product surge vessels 146. The liquid anhydride is thereafter pumped via pump 148 to storage tank 150.
The crude phthalic anhydride liquid is delivered via pump 152, filter 154 and pump 156 to preheater 158 where it is heated to approximately 500.degree. F. The heated crude phthalic anhydride is conveyed continuously through decomposers 160 and 162. The residence time in each decomposer is about 6-12 hours. Decomposers 160 and 162 operate under a slight vacuum (about 700 mm Hg absolute) and high temperatures (e.g., 500.degree. F.) to convert the small amount of phthalic acid that is present to phthalic anhydride. Also, most of the maleic anhydride in the crude is removed by evaporation or chemical reaction. Evaporated impurities especially water are removed via condenser 164 and ejector jets 165. Condensed phthalic anhydride is returned to the decomposers from condenser 164.
Purified phthalic anhydride is pumped from the decomposers via pump 166 to light ends column or fractionation column 168. Column 168 includes reflux condenser 170 and ejector jets (not shown). Low-boiling by-products, e.g., maleic anhydride and benzoic acid, along with a small amount of phthalic anhydride are removed at the top of column 168. Low pressure steam is generated in reflux condenser 170. Crude phthalic anhydride from the bottom of column 168 is fed via pump 172 to either reboiler 174 where it is returned to column 168 or second fractionation column 176.
Pure phthalic anhydride is removed from the top of column 176. Along with the tailings, phthalic anhydride is continuously removed from the bottom of column 176 and sent to reboiler 178 via pump 179 in order to maintain the temperature of the column. Column 176 includes a reflux condenser 180 and ejector jets (not shown). The pure phthalic anhydride from the top of column 176 is sent to phthalic anhydride run-down tanks 182 via pump 184, and eventually stored in storage tanks (not shown).
Various conventional methods for removing water and concentrating phthalic anhydride are set forth in U.S. Pat. No. 3,725,211 (Gehrken et al.), which issued Mar. 1, 1972, U.S. Pat. No. 3,655,521 (Gehrken et al.), which issued Apr. 11, 1972, U.S. Pat. No. 3,507,886 (Suter et al.), which issued Apr. 21, 1970, U.S. Pat. No. 3,397,121 (Fitzgerald), which issued Aug. 13, 1968, U.S. Pat. No. 3,178,452 (Smith et al.), which issued Apr. 13, 1965, U.S. Pat. No. 3,650,906 (Gehrken et al.), which issued Mar. 21, 1972, and U.S. Pat. No. 3,303,203 (Melnstein), which issued Feb. 7, 1967.
Based on recently obtained equilibrium data on the decomposition of the acid together with computer modeling studies, the present inventors have discovered that there can be very little water loss from the system without large losses of phthalic anhydride. Lowering the temperature favors the formation of phthalic acid so the water content of the vapor is depleted by both acid formation and the higher solubility of water in the liquid at the lower temperature. Moreover, the lower temperature frequently leads to plugging the condenser with phthalic acid which has been a major problem in plant operations, often leading to bypassing of the condenser for periods of time, during which losses of phthalic anhydride increase significantly.
The present inventors have discovered that by taking the vapors directly from the decomposer and putting them into the upper tray section of the downstream light ends fractionation column, (preferably just below the column overhead condenser), the decomposer reflux condenser and vacuum system can be eliminated. Moreover, in the fractionation column the water is fractionated away from the phthalic anhydride so that the losses of phthalic anhydride are reduced.
Currently, in order to keep the losses of phthalic anhydride low from the decomposer's reflux condenser, phthalic acid concentration to the column feed would have to increase. This would then increase the water concentration in the overhead section of the column to the same levels as in the present invention. In effect, the method described hereafter by the present inventors allows the water to bypass all but the top section of the fractionation column. The surprising result of eliminating the decomposer condenser and jet ejectors of the decomposer section is that the phthalic acid levels in the decomposer and in the feed to the column are reduced. This in turn reduces the losses of phthalic anhydride and improves product quality by decreasing the level of phthalic acid in the final product.
The present invention also provides many additional advantages which shall become apparent as described below.